The present invention relates to a rotating electric machine having a magnetic circuit which in one of the parts of a rotor and a stator of the machine comprises an element having a slot for a winding having layers of cables extending substantially axially and arranged substantially radially outside each other, said cables comprising an inner conductor comprising a plurality of strands and an insulation externally thereof.
All types of rotating electric machines of the type with winding of a cable are comprised, i.e. such machines in which there is an insulation sheet around the conductor and the conductor is formed by a bundle of strands. All voltage ranges, high voltage as well as intermediate voltage and low voltage are comprised.
The element in the magnetic circuit with a slot for the cables may as mentioned be arranged in any of the parts: rotor and stator, of the electric machine. xe2x80x9cSlotxe2x80x9d is here to be given a broad sense and does not necessarily mean that this is so narrow that the element alone keeps the cables in place.
The electric machine may be arranged to function as generator and/or motor. For the purpose of example it may be mentioned that the machine could be a synchronous machine used as generator for connection to distribution and transmission networks or as motor or for phase compensation and voltage regulation. Other types of machines, such as asynchronous alternating current machines are also conceivable.
The element has a design allowing an alternating magnet flux therein, and it is preferably but not necessarily formed by a magnetic core of laminated sheet being normal or oriented, i.e. thin sheets being mutually insulated, for example through an insulation lacquer so as to keep the eddy current losses in the element on an acceptable low level.
A rotating electric machine of the type defined in the introduction is for example known through WO 97/45919 of the applicant, and it is schematically illustrated in the appended FIG. 1 how an electric machine of that type may be constructed. The element 1 of the magnetic circuit is in this case formed in the stator 2. The rotor with two rotor poles 3, 4 shown (it will in the practice have more, for example four) is designated by 5. The element 1, or actually the stator, is In a conventional way composed by a laminated core of electric sheet successively composed by sector-shaped plates. The number of teeth 7 extends from a back portion 6 of the core located radially outermost radially inwardly towards the rotor. A corresponding number of slots 8 are arranged between the teeth. The slots receive a winding of layers of cables 9 extending substantially axially and arranged radially outside each other. The cables 9 comprise an inner conductor 10 consisting of a plurality of strands and an insulation 11 arranged outside thereof. Since we speak about a high voltage generator and the voltage of the cable layers increases with the distance from the rotor through the connection made here the insulating layers get thicker in the direction away from the rotor. As a consequence of the limited availability of suitable cable dimensions no continuous- decrease of the cable insulation towards the rotor has taken place, but cables having three different dimensions of the cable insulation are used, such as for example for 70 kV, 100 kV and 130 kV.
It is illustrated in FIG. 3 how the magnetic alternating flux generated in the teeth 7 of the element 1 upon rotation of the rotor extends around the cables arranged in the slot 8 in question. A stray flux will as illustrated by dashed lines 12 be created through the conductors in an attempt of the magnetic flux to make a shortcut. This stray or leak flux involves some inconveniences. Firstly, the main flux is reduced therethrough, which results in a somewhat lower power of the electric machine. Furthermore, the stray flux will generate eddy currents in the strands, which results in heat generation and a demand of cooling the cables, which normally takes place indirectly by cooling the sheet package surrounding them. The strands have been electrically insulated from each other for reducing the eddy current losses, so that the magnetic flux experiences thin surfaces when intersecting the conductors of the cables and thereby the eddy currents and accordingly the eddy current losses will be low. However, this means that the conductors and thereby the cable will be considerably more expensive than if the strands had been uninsulated, and the insulation is usually achieved by painting the strands with an insulating lacquer, which means a load on the environment.
The object of the present invention is to provide a rotating electric machine of the type defined in the introduction, in which at least the disadvantage last mentioned of such machines already known has been substantially reduced.
This object is according to the invention obtained by the fact that in such a rotating electric machine a larger share of the strands of the cables closest to the other part of the rotor and the stator are electrically insulated with respect to each other than of the cables most far away from the other part.
Thus, the invention utilizes the understanding that the magnitude of the stray or leak flux through the respective cable depends upon the relationship between the flow path closed through the cable and the alternative flow path around the cables, which means that the stray flux decreases for each cable layer in the direction away from the rotor. It has turned out that it is therefore possible to allow considerably larger continuous surfaces in the cable conductors intersected by the magnetic flux farther away from the rotor than closer thereto and thereby larger eddy current loops, since the stray flux is in any way so much lower that the eddy current losses in the conductors are kept on an acceptably low level. By electrically insulate fewer strands with respect to each other in the cables most far away from the other part, in the case discussed above the rotor, than in the cables closer to this other part, considerable costs may be saved. The costs of a strand with an insulation are normally in the order of twice the costs of a strand without insulation. Furthermore, it is when using an insulating lacquer for the insulation in this way possible to spare the environment by a considerably reduced consumption of lacquer when manufacturing the cable. Accordingly, the Invention is based on the idea to concentrate on reducing eddy current losses where it is mostly needed, i.e. where the leak or stray flux is the highest.
According to a preferred embodiment of the invention substantially all strands are electrically insulated with respect to each other in the cable layer closest to said other part, which is advantageous, since the leak flux is there the highest and the need to keep the surfaces experienced by this leak flux down on a low level is then also the greatest.
According to another preferred embodiment of the invention substantially none of the strands are electrically insulated with respect to the rest of the strands in the cable layer located most far away from said other part. Such an advantageous design of the cable in said cable layer is in fact possible, since the leak flux of that cable layer is very low thanks to the short extra way to go for the main flux around the cable layer in the element with a considerably higher magnetic reluctance.
According to another preferred embodiment of the invention the share of strands electrically insulated with respect to the rest of the strands of the cable decreases in the direction away from said other part. The advantages mentioned above of a lack of electric insulation of the strands with respect to each other where it is in fact not needed is hereby obtained. According to other preferred embodiments of the invention said decrease may take place for each cable layer in the direction away from said other part or stepwise after two or more cable layers having the same proportion or share of strands being electrically insulated with respect to each other in the direction away from said other part.
According to another preferred embodiment of the invention the electrical insulation of the strands with respect to each other is obtained by providing the respective insulated strand with an insulating thin envelope surrounding the strand, which according to an embodiment is formed by an insulating lacquer. According to another embodiment the electrical insulation of the strands with respect to each other is obtained by making such electrically insulated strands of aluminium, the surface of which is allowed to oxidate for forming an aluminium oxide layer surrounding the strand. It is in this way possible to do without the insulating lacquer undesirable from the environmental point of view, and aluminium may then advantageously be used for the strands requiring an electrical insulation with respect to each other and for example copper of the strands with no need to be electrically insulated with respect to each other. Thus, the cable or cables most far away from said other part could advantageously have a conductor formed by strands of copper uninsulated with respect to each other, while the cables closest to the other part could have the conductor thereof formed by strands of aluminium. Accordingly, in the case discussed above of a high voltage generator the 70 kV-cables could for example have strands of aluminium, while the 100 kV-cables could have strands of copper.
According to another preferred embodiment of the invention said winding is at least partially formed by a cable in the form of a flexible electric conductor having an envelope able to confine the electric field generated around the conductor. This enables a reduction of electric losses, which in its turn results in a lower temperature of the cable and the surrounding element, which reduces the need of cooling and makes it possible to construct cooling arrangements possibly existing in a more simple way than without such a design of the cable. The cable may be made in the form of a flexible cable, which means substantial advantages with respect to manufacturing and mounting compared to rigid windings prefabricated and traditionally used until today. Furthermore, the use results in an insulation system with an absence of gaseous and liquid insulating material with the disadvantage adhered thereto obtained in this way.
Further advantages as well as advantageous features of the invention appear from the other dependent claims and the following description.